The present invention relates to a helical filter capable of adjusting the band width.
Generally, a helical filter comprises a plurality of helical coils which are electromagnetically coupled, each having an open end at one end thereof and a grounded end at the other. Specifically, each one of the helical coils is wound around a bobbin in a metal case, and a plurality of the cases are fixed together at their sides by soldering or welding, or the helical coil is wound around a plurality of bobbins arranged on a common base, and the entire system is covered with a case having partitions between each of the helical coils, and some others.
Then, each of the helical coils functions as a resonator, and the resonance frequency thereof is adjusted by changing the distributed capacitance between the helical coil and the case around it. The distributed capacity is changed by moving a dielectric member between the helical coil and the case or by moving in the vicinity of the coil a metal screw which is electrically connected to the case.
The resonance frequency of each resonator is made equal, and the filter is resonated against the input signal of this resonance frequency. Also, the band width of the filter is adjusted in accordance with the coupling condition between the resonators. However, once it is defined by the side of the case between the helical coils or the size of the coupling window of the partition at the time of designing, no adjustment is possible thereafter.
The same is applicable in the case where the band width is defined by the space between the helical coils without a coupling window as a matter of course.
Nevertheless, it often occurs that the band width of an assembled helical filter departs from the band width defined at the time of its design.
Hence, if the band width is rigidly regulated, the yield of the helical filter is significantly reduced.